Thiazolidenediones (TZDs), including troglitazone (TG), rosiglitazone (RG), pioglitazone (PG), and ciglitazone (CG), are synthetic ligands of the peroxisome proliferator-activated receptor γ (PPARγ) (1). This family of PPARγ agonists improves insulin sensitivity by increasing transcription of certain insulin-sensitive genes involved in the metabolism and transport of lipids, thus representing a new class of oral antidiabetic agents. More recently, certain TZDs, especially TG and CG, have also been shown to inhibit the proliferation of many cancer cell lines that express high levels of PPARγ, including, but not limited to, those of colon, prostate, breast, and liposarcoma [review: (2)]. As PPARγ-mediated effects of TZDs promote the differentiation of preadipocytes, one school of thought attributes the same mechanism to the terminal differentiation and cell cycle arrest of tumor cells (3). However, the PPARγ-activated target genes that mediate the antiproliferative effects remain elusive, as genomic responses to PPARγ activation in cancer cells are highly complicated (4). Reported causal mechanisms include attenuated expression of protein phosphatase 2A (5), cyclins D1 and E, inflammatory cytokines and transcription factors (2), and increased expression of an array of gene products linked to growth regulation and cell maturation (4). On the other hand, several lines of evidence indicate that the inhibitory effect of TZDs on tumor cell proliferation was independent of PPARγ activation. For example, the antitumor effects appear to be structure-specific irrespective of potency in PPARγ activation, i.e., TG and CG are active while RG and PG are not. Also, there exists a three-orders-of-magnitude discrepancy between the concentration required to produce antitumor effects and that to mediate PPARγ activation. To date, an array of non-PPARγ targets have been implicated in the antitumor activities of TG and/or CG in different cell systems, which include intracellular Ca2+ stores (6), phosphorylating activation of ERKs (extracellular signal-regulated kinases) (7, 8), JNK (c-Jun N-terminal protein kinase), and p38 (9), up-regulation of early growth response-1 (10), p27kip1 (11), p21WAF/CIP1 (12), p53, and Gadd45 (13), and altered expression of Bcl-2 family members (9). However, some of these targets appear to be cell type-specific due to differences in signaling pathways regulating cell growth and survival in different cell systems.
In light of the potential use of TZDs in prostate cancer prevention/treatment (14, 15), signaling mechanisms whereby these PPARγ agonists inhibit the proliferation of prostate cancer cells represent the focus of this investigation. We report here the development of novel TZD derivatives that lack activity in PPARγ activation but retain the ability to induce apoptosis in two prostate cancer cell lines with distinct PPARγ expression status, suggesting that these two pharmacological activities are unrelated. More importantly, we demonstrate that TZD-mediated apoptosis was attributable, in part, to the inhibition of the anti-apoptotic functions of Bcl-xL and Bcl-2 by disrupting the BH3 domain-mediated interactions with pro-apoptotic Bcl-2 members. From a translational perspective, dissociation of these two pharmacological activities, i.e., PPARγ activation and Bcl-xL/Bcl-2 inhibition, provides a molecular basis to use Δ2-TG as a scaffold to generate a novel class of Bcl-xL/Bcl-2 inhibitors. Accordingly, we developed a structurally optimized Δ2-TG derivative (TG-88) with high in vivo potency in inhibiting PC-3 tumor growth.